Fix/Panic in SessionCreate

[notimetoname]

Do not panic if signer was provided as a default one. Also, return ErrMissingSigner if no signer was provided at all.

[cthulhu-rider]

btw do u expect user.IDFromKey to panic on nil signer? It can return an error

[notimetoname]

btw do u expect user.IDFromKey to panic on nil signer? It can return an error

@cthulhu-rider, to be honest, I do not like getting a user from a signer at all cause signer must sign and it even can be not from our sdk at all. It is not smth unnatural to me when a Signer interface drops a Public() PublicKey cause it does not look like it can break Sign process but it will break user.IDFromSigner in our case. That is where my ideas break.
But that is what we get in RC 8, and the PR should just fix the panic sdk throws, so we can continue the discussion in some other issue.

cc @roman-khimov, @smallhive

[roman-khimov]

signer must sign and it even can be not from our sdk at all

The problem is key->ID mapping. It's tied to Neo's verification scripts and that can only have 256r1 keys.

[notimetoname]

@roman-khimov, if key->ID relation is that strong, i would not get a user from a Signer and return key->ID conversion (i did sdk update and it adds more code, not removes). Anyway, i do not have an answer to @cthulhu-rider's question: "do u expect user.IDFromKey to panic on nil signer?" and need help.

[roman-khimov]

if key->ID relation is that strong

Then we have an option of adding ID() user.ID to Signer itself. It can internally do IDFromKey or whatever else (GetScriptHash()), but it's then left up to the Signer.

do u expect user.IDFromKey to panic on nil signer?

I do.

[notimetoname]

Then we have an option of adding ID() user.ID to Signer itself.

I like it more.